Since optical elements (lenses, mirrors) are one of the core elements of optical devices, their surface accuracy must be ultra-smooth (roughness: Ra is below 1 nm), and they must also have a relatively high surface figure (the shape accuracy is below 0.5 microns), to achieve excellent optical performance. In the LED field, monocrystalline silicon (Si), monocrystalline germanium (Ge), gallium arsenide (GaAs), monocrystalline silicon carbide (SiC), and sapphire (Al2O3) etc., serve as semi-conductor substrate materials, so they must also have an ultra-flat and ultra-smooth surface (roughness of Ra must below 0.3 nm) in order to meet the growth of epitaxial film and, there must be no defects and no damage. Flat optical elements and the semi-conductor substrate both need planarization, but the conventional processes for planarizing flat optical elements and semiconductor substrates are mainly surface grinding, ultra-precision polishing, chemical mechanical polishing, and magnetorheological polishing; this means the quality and precision of the finishing method determines how well optical devices and semi-conductor devices perform.
Magnetorheological finishing is a new method for finishing an optical surface; it was put forward by KORDONSKI and his collaborators in the 1990s, and is based on a combination of electromagnetics, fluid dynamics, analytical chemistry, and processing technology, etc. Magnetorheological finishing is good for polishing and there is no secondary surface damage, so it is suitable for finishing complex surfaces, unlike traditional polishing processes. Magnetorheological finishing has since developed into a revolutionary finishing method for optical surfaces, particularly for finishing axisymmetric aspheric surfaces, so it is widely used in the final processing of large-scale optical elements, semi-conductor wafers, LED substrates, and liquid crystal display panels, etc. However, current magnetorheological finishing used to finish flat workpieces is mainly using the various models of magnetorheological finishing machines developed by QED, a corporation from the United States. These machines work by placing the workpiece above an arc-shaped polishing disc such that a concave gap is formed between the surface of the workpiece and the polishing disc. An electromagnet pole or a permanent magnet pole with an adjustable magnetic flux density is placed under the polishing disc to form a high-intensity gradient magnetic field at the concave gap. As the magnetorheological fluid moves with the polishing disc to a position adjacent to the concave gap formed by the workpiece and the polishing disc, a flexible protruding “polishing ribbon” is formed. However, contact between the “polishing ribbon” and the workpiece surface belongs to “spots” local contact. During the finishing process, only by controlling the “spots” to perform trajectory scanning along the workpiece surface according to a certain rule, can the entire surface be finished. This trajectory scanning process requires a lot of time which means it is inefficient and it is not easy to guarantee an accurate finishing shape.
To improve the efficiency of magnetorheological finishing, Patent No. CN200610132495.9 sets forth an abrasive polishing method based on the magnetorheological effect and a polishing device thereof, which works on the principle of magnetorheological finishing and an action mechanism of cluster; this process has already been carried out in a large number of experimental studies. Although this method forms a regional polishing pad using the cluster method, it is difficult to finish the workpiece uniformly, so following a deep analysis, it is found that due to the viscoelasticity of magnetorheological fluid, the workpiece will press down the protruding flexible polishing pad set forth in the patent and make it irrecoverable when passing by the flexible polishing pad. Thus, the flexible polishing pad loses its pressure on the workpiece, which makes a huge difference between the material removal rate at the edge of a workpiece and that in other areas. Moreover it is difficult to renew the abrasive in the viscoelastic polishing pad which further reduces the finishing effect (as shown in FIG. 1). Therefore, based on this deep research, the present invention has a self-sharpening polishing device with magnetorheological flexible polishing pad formed by dynamic magnetic field and polishing method thereof, which intuitively maintains constant pressure during the finishing process and enables the abrasive to be renewed, whilst simultaneously self-sharpening in real time during this process. This finishing device and finishing method are eminently suitable for high-efficiency ultra-precision finishing of optical elements, semiconductor wafers, ceramic substrates, and other flat materials.